Embodiments of the present invention relate to transponders (made up of transmitter and responder) and methods for operating transponders which may be applied in the so-called RFID technology (RFID=radio frequency identification), for example.
RFID technology is a radio-based contactless identification method. A RFID system typically includes one or several reader devices and a plurality of transponders. Both the reader device and the transponder each comprise an antenna which critically influences a coverage of data communication between the reader device and the transponder. If the transponder comes near the antenna of the reader device, both of them (transponder and reader device) exchange data. Apart from the data, the reader device transmits energy to the transponder. An antenna coil embodied as a frame- or ferrite-antenna, for example, is located in the transponder. For operating the transponder, the reader device generates an electromagnetic alternating field by means of its antenna.
The antenna of the reader device also includes a coil with several windings. If the transponder is brought sufficiently near to the antenna of the reader device, the electromagnetic field of the reader device will generate an induction voltage in the coil of the transponder. This induction voltage may be rectified and limited and serves for voltage supply of the transponder or a load circuit located in the transponder, such as a microcontroller. From the (alternating) induction voltage induced in the transponder, a clock frequency for the load circuit may be additionally derived, which is then available as a system clock for the load circuit in form of a memory chip or a microprocessor.
If the transponder is sufficiently near to the reader device, a minimum field strength required for the operation of the transponder will be exceeded, which in the following is referred to as “response field strength”. The response field strength of the transponder depends upon the energy consumption of the load circuit integrated in the transponder and the properties of the transponder antenna, such as antenna gain or resonance frequency. If the response field strength is exceeded, the transponder will start operating, receive data transmitted from the reader device and perform e.g. an amplitude modulation of the electromagnetic field reflected by its antenna according to the data to be transmitted by the transponder. The reflected electromagnetic field may be received by the antenna of the reader device and then be evaluated in the reader device with regard to the data modulated upon.
As has been previously described, the response field strength of a transponder depends, among other things, upon energy consumption of an integrated circuit, or a load circuit of the transponder. There my be cases in which it is not desirable to achieve a response field strength as low as possible, but rather, in contrast, in which it is strived for a transponder to communicate with the reader device only from a certain predetermined field strength of an electromagnetic field prevailing at the location of the transponder and not below.